This invention relates to the field of remote energy-emitting source location through passive received signal processing.
Although radio locating has been used since the early days of radio, current military apparatus with its limited output signal durations and the availability of computerized signal processing, faster analog to digital conversion apparatus and a need to accomplish rapid, accurate signal locations from a moving vehicle provide opportunity for improvement in this art.
Current state of the art for passive geo-location of a ground based emitter using two aircraft requires each aircraft to independently measure time of arrival (TOA) of the radio frequency (RF) signal, exchange the information, so that one or both can then calculate time difference of arrival (TDOA). This will require atomic clocks on both aircraft, or some other method of determining the precise time at each aircraft. The accuracy of this open loop approach is also limited by the sample times of a sampling clock and the linearity of the pulse leading edge. The open loop method is also limited by the differences in the path lengths from the antenna to the measurement receiver. A more precise method of determining TDOA is a differential approach where TDOA is obtained by iteratively adjusting an analog variable delay line until the two signals cancel. This cancellation is very precise because it not only cancels the pulse envelope but also the RF carrier by phase alignment. The method of TDOA measurement by differential delay adjustment requires both RF signals to be available at the two channel measurement receiver and that both channels operate with a common LO and common sampling clock. This method also includes an approach to calibrate the two receive paths of the interferometer.
The present invention improves the precision of currently available methods by using two aircraft to form a large baseline interferometer. This avoids problems associated with using a single aircraft with one or more tethered antennas. The present invention allows for various configurations for geo-location of ground-based emitters from moving aircraft and tracking both stationary and moving emitters.